Relaxation oscillator



April l 1947. o, o I I 2,418,825

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CK-HJMdiK TIME Patented Apr. 15, 1947 UNITED S A PATENT O'FFiCE.

RELAXATION OSCILLATOR- Application January 8, 1943, Serial No.v 471,661

5 Claims; 1, This .invention relates to, an improvement in relaxation oscillators of the type known as asy metrical multivibrators.

Numerous circuits arekn-own to the prior art. They serve as generators of saw-tooth voltage oscillations varying in frequency from several megacycles to a few hundredths of a cycle per second and the oscillating voltages so generated areused to controlvarious phenomena, for example, the defiectionof. an electron beam in the cathode ray oscilloscope. The voltage wave form ofanasymmetrical'multivibrator as recorded on thea oscillograph exhibits for each oscillation a relatively slow change in one direction between two extremes of, voltage followed by a rapid change in the opposite direction between the same extremes. In United States Patent 1,934,322, granted November '7, 1933, to W. O. O'sbon, there is disclosed a multivibrator circuit of which the asymmetry serves to confine the rapid change of voltage to a small fraction of the oscillation period. Osbon also. discloses thermionic means to insurellinearitywith timeof the relatively slow voltage change. i

The. interval. of rapid voltage change, conveniently called .the retrace interval, is made by such circuits as that of Osbon adequately short in the case ofv higher frequency oscillations, but morewis needed when extremely low oscillation frequencies. are used. The problem solved in the present inventionis toshorten this retrace interval to a substantially negligible time, even for a cycleten seconds long. The solution is found in the use of 1a booster circuit to accelerate the charge of. the sweep condenser of which the periodic voltage variation constitutes the sawtooth wave, while leaving the condensers rate of discharge to .be regulated by the conductances of a thermionic discharge device.

It is therefore an object of the invention to providean improved apparatus for the generation ofsaw-tooth voltage waves.

Another object of the invention is to provide a multivibrator circuit capable of generating sawtoothvoltage .waves having one side of the Wave relatively steeper than is possible with circuits previously known while sacrificing no advantage afiorded by thedisclosures of the prior art.

Another object of the invention is to provide a booster circuit cooperative with and controlled by-aknown form of multivibrator circuit toenable; the latter, to generate saw-tooth voltage wave in which the retrace interval is reduced to a substantially negligible fraction. of the voltage cycle-.1;

These .objects are, attained in a manner which will be clear from thefollowing description of the invention read with reference-to the accompanying drawings in which;

Fig 1 is the. complete circuit of the apparatusof the inventionomitting unessential details;

Fig. 2' symbolically represents voltage pulses determinative of theroperation of the booster circuit ofFig; 1; and- Fig; 3' exhibits oscillographic comparisons ofthe retrace intervals of the conventional multivibrator circuit and of-thecircuit of-the invention.

InFig. 1 numeral l indicates within the-dashdotline a, multivibratorcircuit the design of which is" substantially conventional. Battery at,

the negative "terminal of which isgrounded sup plies voltage to anode it of tube V! through resistors 5'! and-tain series, and to anode 59 of tubeVZ- through resistor till;

Vl' throughresistorstl and 62and--directlyto screen grid '63; tubeV'A. Condenser t l is con-- nected between ground and the junction of resistor 62 with. grid Bl. through resistor 68 to anode I8 is maintained constant byvoltage regulator tube-65, a VRl-30 asindicated in thafigure. Anode st of tube V2 is joinedto controlgrid 56 of tube Vi through condenser 61in series with resistor--68; the-junction of the two lastnamed 'elementsbeing grou'nded through resistor '69. In tube VI, cathode 5 and suppressor, grid 10* aredirectly joined as are cathode 2 and deflector plates ll in tube V2. Cathode 5 of tube VI isgrounded through fixed resistor 4 in series with the portion of potentiometert selected by adjustment oftap 12. In-' eluded between: ground and cathode 2 of tube is condenser C of capacity approximately 33 microfarads as shown. Anode It of tube VI and controlgrid l8: of tube V2 are connected through resistor ll; conductorfizfleads from the junction ofanode 16 with resistor ll tothe booster circuit later. described, while conductor 6 leads from the junction of cathode-2 with condenser C to anode l of tube V3of a discharge circuit later described in detail and. to cathode 52 of gas-filled tube-V6 forming a partof the booster circuit. Vacuum Frornbattery voltage is alsosupplied to screen grid SI of tube The voltage supplied i 3 bilized by cathode feedback derived from variable potentiometer 3 and fixed resistor i in series between ground and cathode of tube VI. Numerical examples of the constituent elements are given in the appropriate places in the diagram of Fig. 1 whereby the circuit may be constructed. Details of cathode heater circuits are in every case omitted.

In the operation of the circuit of Fig. 1 tubes VI and V2 are alternately conducting but for unequal time intervals. Trigger tube V2 conducts only during the interval of charge of condenser C and only during this interval does space current flow in tube V2. Tube VI is conducting during the discharge of condenser C through the circuit of tube V3 later to be described. The retrace interval of the circuit which it is the object of the invention to shorten is the interval of charging condenser C. This condenser begins to discharge at the moment V2 ceases to be conductive and the discharge takes place through a constant current tube V3, preferably a pentode such as the 6AC'7. The junction of one terminal of condenser C and cathode 2 is connected by conductor 6 to anode 'I of tube V3; the other terminal of condenser C is grounded. Conductor 5 is continued as shown to constitute output terminal III of the circuit. Suppressor grid 8 and control grid 9 of V3 are together connected to the negative terminal of 150 volt battery Iii, the positive terminal of which is grouned. Battery I3 is shunted by by-pass condenser II. Screen grid I2 of tube V3 is connected to ground and to the positive terminal of battery II] and this connection is continued to become the other output terminal 82. In series between ground and cathode I5 of tube V3 are connected fixedresistor I3 and variable potentiometer I4. Discharge tube V3 with its associated elements forms actually a part of the multivibrator circuit. Solely for con venience in description, tube V3 and its accessories are shown in Fig. 1 outside the dot-dash line I.

Under the circumstances indicated in the figure, tube V3 is continuously conducting. During the conducting interval for tube V2 condenser C varies in potential from 75 to 150 volts, approximately, at a rate determined by the constants of multivibrator I and this voltage rise is available across terminals Ill, 02.. When tube V2 ceases to conduct, condenser C discharges through the anode-cathode path of tube V3 at a slower rate determined by the setting of the potentiometer I4. This decreasing voltage in condenser C may likewise be impressed on a subsequent circuit connected to the output terminals.

If such a sequence of voltage variations is used, for example, to control the electron beam of the cathode ray oscilloscope the relatively slowly decreasing voltage in condenser C will furnish the sweep and the rapid recharge of that condenser the retrace. The frequency of the saw-tooth oscillation of the voltage of condenser C is determined by the setting of potentiometer I i which fixes the interval of discharge together with the constants of multivibrator I which fixes the interval of recharge. For example, with 32 microfarads as the capacity of condenser C and suitable adjustment of potentiometer It the OSCll-r lations in voltage will have a frequency of .15 cycle per second. The voltage across terminals GI, I32 will require about 28 milliseconds to rise from '75 to 150 volts, declining to '75 volts in the ensuing 6.64 seconds. Changing the value of condenser C to 3.2 microfarads without change in 4 conductance of V3 will change the oscillation frequency to about 1.5 cycles per second with a retrace interval of some 3 milliseconds. These conditions of the conventional operation areshown in 3 where A and C are oscillographic records of retrace for the oscillations of frequencies .15 and 18 cycles per second, respectively. A thousand cyclev timing wave is shown with each oscillogram. In the case of C, potentiometer It has been reset. In each case, the charge interval is fixed by the capacity of condenser C.

To reduce this charge interval in the ratio of three or four to one with the same oscillation frequency requires the addition of the booster circuit enclosed within the dash-dot line 2I of Fig. 1. Anode E6 of tube Vi is joined through .i megohm resistor Ii to control grid I8 of tube V2. In the operation of multivibrator I, at the moment tube V2 becomes conducting a positive voltage pulse approximately square-topped appears on grid I8 and on anode I3. The duration of this pulse is the charging interval of condenser C and the pulse ceases when G is charged to volts.

This positive voltage pulse, represented by a in Fig. 2, is transferred by conductor 22 to the booster circuit 2i of Fig 1. Conductor 22 branches to control by way of conductors 23 and 24 tubes VA and V5, respectively. Tube V4 may suitably be a 6AC7, VS a gas triode such as the 884. Conductor 23 transmits positive pulse a of Fig. 2' through condenser 25 and resistor 26 to control. grid 21 of tube V4. Resistor 28 joins to ground. the junction of condenser 25 and resistor 26 Tube V3 is thus an amplifier resistance-capacity coupled to tube Vi. In V4, cathode 29 is grounded through .i megohm biasing resistor 33 by-passed by condenser 3i; suppressor grid 32 is grounded; screen grid is connected through resistor 34 to the positive terminal of 300 volt battery 35 of which the negative terminal is grounded. Grid 3'2 is directly grounded and capacitively coupled to grid 33 by condenser 33 and anode 31 is supplied through resistor 38 from battery 35. The circuit constants of booster 2i are later tabulated. Underthese conditions, tube V4 is conducting and the positive pulse of anode I5 at VI transferred to grid 2'! of V 3 is reversed with approximately unit gain in V4 to appear as a negative pulse at anode 31. This reversed pulse is shown at b in Fig 2. In this figure voltage and time scales are arbitrary and the diagrams are illustrative only.

At the same time that pulse a app-ears at grid 21 it also is transferred through resistor 39 to control grid d8 of tube V6- Reverse pulse 1) on anode 31 is diiierentiated by the circuit comprising condenser 3 i resistance 42 and 30 volt battery d3 of which the positive terminal is grounded as shown in the figure. To the junction of condenser ll andresistor :32 is connected control grid 44 of tube V5, suitably a GLGG like tube V2. Deflector plates 45 and cathode d6 of V5 are togethergrounded while anode 5'1 is supplied through resistor d8 from 300 volt battery 49, which directly supplies screen grid 53. Battery is likewise supplies anode 5| of V6 in parallel with anode 4''! of V5. Cathode 52 of V6 is connected through conductor 55 with conductor 6 leading to anode 'l of discharge tube V3 and output terminal OI The operation of the circuit above described is as follows:

Positive voltage pulse a of Fig. 2 appear on anode I6 of VI and control grid I8 of V2 and is at once eflfective on control grid 21 of V4 and grid 40 of V5. The latter tube, V6, at once breaks down allowing the potential of 300 volt battery to flow through resistor 48 to charge sweep con denser C. Condenser C already charged to" "7 5 volts at once charges rapidly to a potential of 150 volts, at which potential tube V2 ceases to be conductive on thecessation of voltage pulse a. Under these circumstancesthe rise in potential of condenser Cfrom 75 to 150 volts takes place with great rapidity.

It is of course requisite to annul the conductivity of V6 when condenser C has acquired the desired potential corresponding to cut-clifof tube V2. This oflice is performed by the differentiating circuit of condenser 41; in series with resistor 42 and 30 volt biasing battery 43. The voltage and poling ofbattery 43 is such as to maintain tube V-5 init'ially blocked.

Positive pulse a; reversed in V4 to appear as negative pulse b on anode 31, is differentiated by the circuit 4|, 42 and appears on control grid 44 of V5 as a pulse represented by c in Fig. 2. Corresponding to the leading side of pulse 22 pulse has a steep and nearly instantaneous negative peak. This peak drives V still further past cutoff but is elsewhere not felt, V5 being already blocked. At the moment the potential of condenser C has reached 150 volts and tube V2 blocks, the cessation of pulse b is represented in c by a steep positive peak. This nearly instantaneous positive peak more than overcomes the negative bias furnished by battery 43 and renders V5 suddenly conductive. The abrupt onset of conductance of V5 is accompanied by an equally abrupt drop in potential of anode 4'! and of anode 5|. By proper choice of circuit constants this drop of potential in anode 5| is made suflicient to extinguish tube V6. Condenser C is now charged to 150 volts, tube V2 is blocked and tube V| begins to conduct as condenser C discharges to volts through the discharge tube V3 at a rate determined by the setting of potentiometer l4.

Except for booster circuit 2| charging of condenser C would have been accomplished by the space current flowing in tube V2. When condenser C is of capacity 32 microfarads, this charging by the multivibrator circuit alone requires 28 milliseconds as shown in A of Fig. 3. Supercharging by the aid of booster circuit 2| reduces the required interval to 8 milliseconds as shown in B, Fig. 3. Fig. 3 shows also at C the 3 millisecond retrace time of multivibrator circuit I when condenser C has the capacity 3.2 microfarads. For this condition supercharging by circuit 2| reduced the retrace time to .8 millisecond as shown by D of Fig. 3.

Appropriate values of the multivibrator are indicated in Fig. 1 and are not identified by reference numerals further than necessary to an understanding of the cooperation of the multivibrator circuit and the booster circuit. For the latter suitable values of capacity and resistance are as follows:

The val es tabulated ta e ar -j a pro riate in the particular booster circuit 2! shown in Fig. 1

in whichtubes v l, V5 and V6 are respectively a 6AC7, a 6L6G and an 884. Ifit is desired to use other tubes than those mentioned for V4: and V5 the corresponding circuit constants maybe revised, it being only necessary to insurej that the positive voltage pulse derived from anode it shall after reversal in V5 and differentiation by the circuit 4|, 42 result in a negative pulse on anodes l! and 5| suflicient to annul the conductance of V5. The temporary conductivity of tube V6, conferred by a first positive impulse directly received from anode l6 and annulled by a second biasing impulse indirectly occasioned by the first impulse, permits accelerated charge of sweep condenser C in the multivibrator circuit thereby accomplishing the object of the invention.

What is claimed is:

1. In a relaxation oscillator including a condenser, means including thermionic devices for alternately charging and discharging said condenser coupled with additional means including thermionic devices controlled by said first-mentioned means for accelerating the charging of said condenser.

2. In combination, means for the generation of electrical oscillations including a multivibrator circuit comprising a condenser, a discharge circuit for said condenser and a pair of thermionic vacuum tubes cooperating to generate a recurrent saw-tooth voltage across said condenser by the alternate charging and discharging thereo-f, and means including thermionic devices controlled by and cooperative with said multivibrator circuit to accelerate the charging of said condenser during each recurrence of said voltage.

3. In a relaxation oscillator, in combination, a multivibrator circuit comprising a condenser, a first and a second thermionic tube, a discharge circuit including a third thermionic tube in shunt with said condenser, each of said tubes having at least a cathode, a control grid and an anode, power supply for said tubes, coupling including resistance and capacity in series between the control grid of said first tube and the anode of said second tube and conductive coupling between the anode of said first tube and the control grid of said second tube, whereby said condenser is alternately charged and discharged between desired extremes of voltage, and an auxiliary circuit for accelerating the charging of said condenser comprising a fourth, a fifth and a sixth auxiliary thermionic tube, each of said auxiliary tubes having at least a cathode, a control grid and an anode, power supply for said auxiliary tubes, coupling including capacity and resistance in series between the anode of said first tube and the control grid of said fourth tube, conductive coupling between the anode of said first tube and the control grid of said sixth tube, a difierentiating circuit coupling the anode of said fourth tube and the control grid of said fifth tube, conductive coupling between the anode of said fifth tube and the anode of said sixth tube, and conductive coupling between the cathode of said sixth tube and the cathode of said second tube.

4. For a multivibrator including a pair of thermionic tubes having each at least a cathode, a control grid and an anode cooperating to generate a recurrent voltage across a condenser in the cathode circuit of one of said tubes by the alternate charging and discharging of said condenser, an auxiliary circuit for accelerating the charging of said condenser comprising a first, a second and a third auxiliary thermionic tube, each of said auxiliary tubes having at least a cathode a control grid and an anode, conductive coupling between the control grid of said one tube in said multivibrator and the control grid of said third auxiliary tube, coupling including capacitance between the control grid of said one tube and the control grid of said first auxiliary tube, a differentiating circuit coupling the anode of said first auxiliary tube and the control grid of said second auxiliary tube, conductive coupling between the anode of said second auxiliary tube and the anode of said third auxiliary tube and conductive coupling between the cathode of said third auxil- 15 8 toothlvoltages are generated between the terminalsof said condenser by alternate charging and discharging thereof, and an auxiliary cir-' cuit additional to said multivibrator circuit in-' cluding thermionic devices controlled by said multivibratorcircuit to accelerate the charging of said condenser.

' OLLE D. ENGSTROM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,126,243 Busse Aug. 9, 1938 2,114,938 Puckle n Apr. 19, 1938 

